In a polymer having a conjugated double bond (a π-conjugated polymer), the polymer itself does not show conductivity, but conductivity is exhibited by doping an appropriate anion molecule and it becomes a conductive polymer material (a conductive polymer composition). A (hetero)aromatic polymer such as polyacetylene, polythiophene, polyselenophene, polytellurophene, polypyrrole, polyaniline, and a mixture thereof, and the like, has been used as then-conjugated polymer, and a sulfonic acid-based anion has most frequently been used as the anion molecule (a dopant). This is because the sulfonic acid which is a strong acid interacts with the π-conjugated polymer with good efficiency.
As the sulfonic acid-based anion dopant, sulfonic acid polymers such as polyvinylsulfonic acid and polystyrenesulfonic acid (PSS), have widely been used (Patent Document 1). In addition, in the sulfonic acid polymer, there is a sulfonated perfluoroalkyl vinyl ether represented by Nafion®, which has been used for fuel cell applications.
The polystyrenesulfonic acid (PSS) which is a sulfonic acid homopolymer has high efficiency in doping to the π-conjugated polymer since the sulfonic acid is continuously present in the monomer unit in the polymer main chain, and it can improve dispersibility of the π-conjugated polymer after doping in water. This is because hydrophilicity is retained by the presence of sulfo groups excessively present in the PSS and the dispersibility in water is dramatically improved.
The polythiophene using the PSS as a dopant can be handled as a highly conductive and aqueous dispersion, and thus, it is expected as a coating type conductive film material to be used in place of ITO (indium tin oxide). However, the PSS is a water-soluble resin, and hardly dissolved in an organic solvent. Accordingly, the polythiophene using the PSS as a dopant has also high hydrophilicity, but its affinity to an organic solvent or an organic substrate is low, and it is difficult to disperse in an organic solvent, and to form a film on an organic substrate.
In addition, when the polythiophene using the PSS as a dopant is used, for example, in a conductive film for organic EL lighting, hydrophilicity of the polythiophene using the PSS as a dopant is extremely high, so that a large amount of moisture likely remains in the conductive film, and the formed conductive film is likely to take in moisture from the external atmosphere. As a result, the luminescent material of the organic EL is chemically changed to lower its luminescent ability, and moisture condenses with a lapse of time which becomes a defect, whereby there is a problem that the lifetime of the whole organic EL device is shortened. Moreover, the polythiophene using the PSS as a dopant has a large particle size in the aqueous dispersion, so that there are problems that unevenness at the surface of the film after formation of the film is large, and a non-light emitting portion called dark spot is generated when it is applied to the organic EL lighting.
In addition, the polythiophene using the PSS as a dopant has absorption in the blue region around a wavelength of 500 nm, so that when the material is used by coating onto a transparent substrate such as a transparent electrode, and when the conductivity necessary for the device to function is supplemented with the solid content concentration and the film thickness, there is also a problem that the transmittance as a member is affected.
In Patent Document 2, it has been proposed a conductive polymer composition formed by a conductive polymer which contains a π-conjugated polymer formed by a repeating unit selected from thiophene, selenophene, tellorophene, pyrrole, aniline, and a polycyclic aromatic compound, and a fluorinated acid polymer which can be wetted by an organic solvent, and 50% or more of which is neutralized by a cation, and it has been shown that it becomes an aqueous dispersion of a conductive polymer by combining water, a precursor monomer of the π-conjugated polymer, a fluorinated acid polymer, and an oxidizing agent in an arbitrary order.
However, in such a conventional conductive polymer, particles are aggregated in a dispersion immediately after synthesis, and when an organic solvent which becomes a highly conductive agent as a coating material is added, aggregation is further promoted and filterability thereof is worsened. On the other hand, if spin coating is carried out without filtration, there are problems that a flat film cannot be obtained due to the influence of particle aggregates, resulting in causing coating failure.
In addition, the polythiophene using the PSS as a dopant can be also used as a hole injection layer. In this case, a hole injection layer is provided between a transparent electrode such as ITO, and a light emitting layer. Since conductivity is ensured by the lower transparent electrode, it is not necessary for the hole injection layer to have high conductivity. For the hole injection layer, it is required to generate no dark spot, or to have high hole transporting ability.